Dripping thermal expansion relief value

Correct me if I get my terminology wrong. I believe I have a dripping thermal expansion relief value on my hot water heater. My precious and quite costly BTUs are going down the drain
The main line has a pressure reducing value. Does it look like I need a thermal expansion tank? Should I pressure test the system?

Good idea. However that will be hotter than those other places. Maybe put a garden hose in the path if you are concerned about overheating the gauge. The downside of that the flexible might be that the hose could serve as bit of an expandable pressure tank, messing up your test. Maybe the packaging on the gauge will indicate a temperature range.

The gauge will handle being on the tank's drain but flush the thing first, or you will probably clog the gauge with sediment when you open the drain valve. It's you that may have problems with the heat, so would need gloves or pliers to maybe install or remove it.

You can often pick up an adapter for a sink aerator, take the aerator off, then screw in the adapter then install the pressure gauge to it.

The cable's in the way, is that a T&P valve on the top of the WH? It should be. It will have a test, release lever on the top and probably a tag.

A pressure relief valve inline to a WH without an expansion tank is a problem waiting to happen.

I do not know how common closed supply systems are in Canada, but if you have one, and do not have an expansion tank, you should add one, otherwise, there is no way you'll ever stop that valve from releasing pressure while the WH is running...water expands when it is heated, and if there's no place for it to go (a closed system means it can't push back out into the supply, which is one reason they make them, to protect the water supply to others in case yours became polluted some way), the pressure WILL rise, and the pressure relief valve is just doing what it should be. An expansion tank, properly precharged, prevents that pressure rise. You'll need a pressure gauge to determine the proper precharge on an expansion tank. With one, you could probably get rid of the pressure relief valve, and install the expansion tank in its place.

With one, you could probably get rid of the pressure relief valve, and install the expansion tank in its place.

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You want to re-think that suggestion! Do not get rid of the relief valve.

If you install an expansion tank, there is no technical reason that it has to be installed at the water heater. It could be on any water heater line that does not have a valve between it an the water heater. I said "technical reason" because I don't know what the codes say about that.

The WH has a pressure relief valve and it is 1/2 the pressure designed for the tank...if your house pressure is excessive, get a PRV, otherwise, you'll be dripping water all the time, or maybe gushing water all the time, depending on the incoming pressure. Those valves generally, are designed for unusual situations, and once they open, they don't always reseal depending on their age and the water conditions...just trouble waiting to happen.

Code most places requires the expansion tank to be between the WH and any cutoff valve to it on the cold side.

I'm getting mixed up with the PRV abbreviations. Is that pressure reducing value or pressure relief value? The main city inlet does have a pressure reducing value, although logic would indicate I shouldn't need to mess with it if the cold water pressure is ok. 80psi about right?

The tank is less then 2 years old but our water does have a high mineral content. I really don't want that dripping relief valve to clog up as a result.

PRV = Pressure Reducing Valve in this case. If you have one, yes it will knock the incoming pressure down to say 80 psi. The problem is that when your WH heats the water, the water expands and the pressure will increase. If you didn't have a PRV (or a check valve), the excess pressure would just push back into the city main (so the pressure wouldn't go up).

The gauge that you get will probably have a red needle that records the highest pressure measured. Leave the gauge on at least overnight and see what the peak is. Also, the gauge doesn't need to be connected to the hot side. The pressure will be felt everywhere in the house. So, you could connect the gauge on an outdoor hose bib (assuming that the bib connects after the PRV). Otherwise, the drain on the WH will do the job.

PRV = Pressure Reducing Valve in this case. If you have one, yes it will knock the incoming pressure down to say 80 psi. The problem is that when your WH heats the water, the water expands and the pressure will increase. If you didn't have a PRV (or a check valve), the excess pressure would just push back into the city main (so the pressure wouldn't go up).

The gauge that you get will probably have a red needle that records the highest pressure measured. Leave the gauge on at least overnight and see what the peak is. Also, the gauge doesn't need to be connected to the hot side. The pressure will be felt everywhere in the house. So, you could connect the gauge on an outdoor hose bib (assuming that the bib connects after the PRV). Otherwise, the drain on the WH will do the job.

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I assume the thermal expansion from the heater will cause the pressure to back up the cold line, but it would peak out at whatever the relief value that's leaking is designed for. Is that accurate enough? Lucky hydraulics are so predictable.

Code most places requires the expansion tank to be between the WH and any cutoff valve to it on the cold side.

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That makes sense, except that I had somehow got the impression that a "heat stop nipple" on the cold water line to the water heater could act as a one-way valve. That would be a problem. I guess my impression was wrong. An expansion tank on the cold water supply would certainly be more heat-efficient.

I assume the thermal expansion from the heater will cause the pressure to back up the cold line, but it would peak out at whatever the relief value that's leaking is designed for. Is that accurate enough? Lucky hydraulics are so predictable.

Yes, the bib is after the PRV, and I have one in my garage. Handy!

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Correct. Without a check valve or some other physical way to separate the hot and the cold, the pressures everywhere will be equal (with some changes up/down depending on which floor you are measuring from). The pressure travels at the speed of sound (nearly 5000' per second in water), so a pressure change at one point is felt everywhere nearly instantaneously.

Water is considered incompressible, so as it expands within a fixed volume, the pressure increases rapidly (just like how water lines can rupture during a freeze). The pressure increases until the set point of the relief valve. The valve opens and dribbles some water out and the pressure comes back down.

As I mentioned, leave it on at least overnight. This is when it usually peaks because you aren't using any water. Also, if you didn't have a PRV, the incoming pressure may also be higher during that time (other people not using water or perhaps the water utility is filling their water towers).

You want to re-think that suggestion! Do not get rid of the relief valve.

If you install an expansion tank, there is no technical reason that it has to be installed at the water heater. It could be on any water heater line that does not have a valve between it an the water heater. I said "technical reason" because I don't know what the codes say about that.

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That is a thermal expansion relief valve and is in place of an expansion tank, so absolutely remove it if you install the tank. It is doing the job it was designed for but it is a waste of water.

The T&P on the WH is required...it opens at MUCH higher temperatures and pressures to ensure the WH doesn't rupture. A steam explosion can literally flatten a house...you want it to work! A residential system, by code most places, says you need a pressure-reduction valve if your water exceeds 80psi. No idea what your pressure relief valve is set to, but it must be higher than your nominal max pressure, or you'll be wasting water all of the time to relieve it. Without an expansion tank, that relief valve is keeping your pressure down. A PRV may have an internal bypass, but it can only open when the house pressure exceeds the incoming water pressure - this means your pressure is not stable, and higher than code. Not all PRV's have bypass valves, either.

You do not need that relief valve. When the PRV starts to malfunction, it could help, but by then, it may no longer work. It could be left in, but they usually are more trouble than they are worth. The expansion tank just provides a place for that expanding water to go. Because it is compressing air, the amount of pressure rise is small for the volume of water needed, and your overall pressure will be pretty stable as long as the PRV is working and the ET is not waterlogged.

Jerome, you're still mixing terms. Every water heater come equipment with a temperature/pressure safety valve, commonly referred to as a T/P valve. It will trip if the temperate or pressure reaches the danger points. Water heaters can and do explode, and I mean big time explode if this valve does not open. If your incoming water pressure is excessive, that's 80 psi or more, you need a pressure regulating valve, commonly called PRV. This will lower the pressure to a safe level. The PRV does have a slight downside. Most of them contain a back flow feature. I'll come back to that feature in a minute. We all know that heated water expands and it does not compress. This means when you water heater starts to heat, temperature builds up quite rapidly. If you do not have a PRV, the expansion can be absorbed by the city water main and you have no problem. However, with the PRV, the expansion has to find somewhere to go, and the back flow feature prevents it from get to the city main. It might find a weak toilet valve, but often it will trip the T/P safety valve. The cure for this is with a Thermal Expansion Tank that is installed in the incoming cold water line between the PRV and water heater. This tank provides a temporary escape route for the expansion. It is air charged to match the PRV setting. Hope this clears things a bit for you.

That extra pressure relief valve that was leaking was an indication that the house pressure was too high OR there was no accommodation for expansion when the WH ran. Once you have an expansion tank in there, you do not need that relief valve (do NOT confuse this with the T&P valve on the WH - that MUST stay). Most people find that the pressure relief valves tend to start leaking as they age, even if the pressure isn't too high, so they're kind of a nuisance. You CAN keep it if you want, and it would be another level of protection. The vast majority of houses do not have one, and work just fine. FWIW, the T&P valve can also start to leak as it ages, especially if you have hard water and it gets tripped because of no expansion tank. You should test the thing annually, but as it ages, that may cause it to not seal after the test. One way to look at that, is it probably should have been changed anyways. Sometimes, the tank fails before the valve does.